1. Field of the Invention
This invention relates to a semiconductor integrated circuit device and more particularly to a semiconductor integrated circuit device containing a nonvolatile semiconductor memory device.
2. Description of the Related Art
For example, a NAND type nonvolatile semiconductor memory device is known as a nonvolatile semiconductor memory device having two-layered gate electrodes of floating gates and control gates.
The NAND type nonvolatile semiconductor memory device has a memory cell array in which a drain-side selection transistor STD and source-side selection transistor STS are arranged on both ends of a series connected circuit of memory cell transistors MC. The selection transistor STD is electrically connected to a bit line via a bit line contact electrode and the selection transistor STS is connected to a source line via a source line contact electrode.
The bit line contact electrode and source line contact electrode are formed by forming contact holes in an inter-level insulating film and filling a conductive material into the contact holes. However, in the photolithography process performed when the contact holes are formed, the contact holes penetrate into the element isolation region in some cases. This is caused by misalignment of the photomask. If the contact holes thus penetrate into the element isolation region, the bit line contact electrode or source line contact electrode is brought into contact with the semiconductor substrate, for example. As a result, a leak current increases to make the device faulty.
Therefore, in order to suppress the device from becoming faulty even when misalignment occurs, a barrier insulating film formed of a silicon nitride film is formed between the semiconductor substrate, the element isolation region and the inter-level insulating film as an etching stopper.
However, the silicon nitride film used as the barrier insulating film contains a large amount of hydrogen and tends to cause charge traps. After formation of gate electrodes, the silicon nitride film used as the barrier insulating film is formed on each of the surfaces of the gate electrodes, semiconductor substrate and element isolation region. Therefore, the characteristic of a memory cell transistor may be deteriorated in some cases by an influence of charges, for example, electrons trapped in portions of the silicon nitride film which are formed on the diffusion layers formed in the semiconductor substrate between the gate electrodes and the gate electrode side walls. In order to improve the deteriorated characteristic, the technique for forming insulating films such as silicon oxide films between the gate electrode side walls, diffusion layers and silicon nitride film is described in Jpn. Pat. Appln. KOKAI Publication No. 2001-148428.
Further, as the elements are further miniaturized, the influence by the parasitic capacitances between the floating gates and between the control gates becomes larger and the influence is given to the transistor characteristic. When the parasitic capacitance between the floating gates becomes large, a variation in the threshold voltage of a memory cell transistor caused by an influence by a variation in the amount of charges stored in the adjacent floating gate becomes large, thereby making it difficult to control the threshold voltage. Further, when the parasitic capacitance between the control gates becomes large, the wiring delay caused when the control gate is driven becomes large, thereby making the operation speed low.
In order to solve the above problems, it is effective to make small the dielectric constant of the insulating films filled in between the floating gates and between the control gates. For this purpose, it is preferable to completely fill materials such as silicon oxide films having small dielectric constants in between the floating gates and between the control gates. The technique is described in Jpn. Pat. Appln. KOKAI Publication No. 2002-280463.
However, if the amount of buried silicon oxide films increases, a trapped amount of charges, for example, electrons increases more depending on the film quality of the silicon oxide film in comparison with a case where silicon nitride films are filled.
In order to improve the above situation, it is preferable to fill the gap between the gate electrodes with a silicon oxide film having a less hydrogen content and a small amount of charge traps. However, in this case, it is necessary to perform the silicon oxide film forming process at high temperatures for a long period of time. As a result, the manufacturing cost will rise and miniaturization of elements becomes difficult.